The Ethernet protocol was initially devised to be used in local area networks (LAN), but its success has been such that it is now used also to implement metropolitan area networks (MAN), and even wide area networks (WAN), including the use of satellite links. In the latter case, terminals communicate with one or more gateways via a satellite, by exchanging Ethernet frames encapsulated in lower level packets, for example DVB (digital video broadcast) packets.
Each Ethernet frame—and each DVB or equivalent packet—comprises a “payload” and a header, the latter notably comprising an origin address and a destination address. The payload of the DVB packet consists of the Ethernet frame, including its header. Thus, each network layer adds a header to the data that is exchanged, which creates an overhead.
Because of their large size, the Ethernet headers generate a significant overhead, leading to a strong degradation of the capacity for level 2 interconnect solutions, notably used for virtual private networks (VPN), collection or backhauling networks and numerous wired or wireless standards. This overhead constitutes a brake to the deployment of native Ethernet transport solutions in satellite networks.
Methods for suppressing the headers are known; one example that can be cited is the “PHS” (payload header suppression) method. This method comprises the following steps: first of all, the “context” of a packet, or frame, which has to be transmitted is identified, that is to say a set of information making it possible to define a flow to which said packet belongs. This context univocally determines certain fields of the header of the packet (origin address, destination address, packet length, etc.), called “static” because they are common to a large number of packets exchanged (all those which relate to one and the same flow); then, an identifier is associated on a one-to-one basis with each so-called context (PHSI: “PHS identifier”). Then, the “static” fields of the header are suppressed, and replaced by just the context identifier. Thus, the header is completely or partly suppressed before transmission, and replaced by a more compact identifier, which results in a great reduction in the overhead.
On the reception side, the suppressed fields of the header are reconstructed from the context identifier. However, in the case of a terminal-gateway link, and even more particularly when such a link forms part of a satellite system, the number of flows—and therefore of different contexts—is very high. This means that the context identifiers must include a minimum number of bytes (generally 3 bytes, making it possible to identify more than 16 million flows), which limits the gain of efficiency which can be obtained. Consider for example the case of a voice packet having a payload of 20 bytes and a DVB header of three bytes; the overhead of three additional bytes introduced by the context identifier represents 13% of the bit rate.
There are also header compression techniques, based on a differential coding (for example ROHC, which stands for “Robust Header Compression”. These techniques are more sophisticated than simple header suppression by notably making it possible to compress certain dynamic fields; however, they have been developed only for level 3 stacks (network layer, for example IP) or higher, and do not therefore include compression of the Ethernet headers.